The present invention relates generally to a Radio Frequency Identification (RFID) tags and systems employing same. More specifically, the invention relates to a RFID tag and corresponding system employing same triggered by a fixed multiple frequency signal. A method for operating the RFID tag is also disclosed.
Radio Frequency Identification (RFID) is becoming an important identification technology in applications such as inventory management, security access, personnel identification, factory automation, automotive toll debiting, and vehicle identification, to name just a few. RFID systems utilize an RFID transmitter-receiver unit (usually referred to as a base station or interrogator) to query an RFID transponder or tag which maybe located within a maximum distance from the interrogator unit. The RFID tag detects the interrogating signal and transmits a response signal containing encoded data back to the receiver.
RFID systems provide identification functions not found in identification technologies such as optical indicia, e.g., bar code, recognition systems. For example, RFID systems may employ RFID tags containing read/write memory of several kilobytes or more. The RFID tags may be readable at a distance and do not require direct line-of-sight view by the reading apparatus, e.g., the base station or interrogator. Furthermore, several such RFID tags may be read by the RFID system at one time, if techniques and/or methods for data collision avoidance, as discussed more fully below, are implemented in the RFID system.
Current developments in the area of RFID tagging and tracking systems have generally evolved into two distinct technology fields, i.e., Near Field Devices and Far Field Devices. Both types of devices employ some common architecture. For example, both types of system employ the interrogator and at least one transponder tag. The interrogator transmits a query signal to the tag and receives data transmitted from the tag. The architecture of each tag generally employs an antenna, a rectifier circuit, and a state machine for modulating-transmitted data. One example of a state machine is a logic circuit that transitions, i.e., steps, through a series of predetermined output states as a series of pulses is applied to the tag circuitry. A clocking circuit of some description usually applies these pulses.
Near Field RFID tags generally employ the simplest circuits. Typical RFID tags have a patch antenna, diode detection for triggering a response, and transmission circuitry for sending data back to the interrogator. Transmission circuitry for this RFID tag usually includes the state machine and a transistor device, the latter being used to change the impedance of the tag's antenna. Most of these RFID systems use a technique called back scattering. The operation of a system based on this technique uses an un-modulated, single frequency continuous wave (CW) signal sent by the interrogator. This CW signal triggers or “wakes-up” the transmitter on the tag. The tag then modulates the reflection of this signal by changing the impedance of the antenna in the RFID tag in accordance with a code supplied by a state machine. It will be appreciated that these systems generally have very limited range. It will also be appreciated that this is done to prevent “collision” of data between two or more tags in close proximity.
It should be mentioned that the RFID tags in a near field RFID system may or may not have a battery. Those without a battery are known as “passive” tags. Simpler systems employ passive tags.
In contrast, the tags employed in a Far Field RFID system have developed around the use of microprocessors and software algorithms. In operation, the interrogator sends a modulated RF signal to the RFID tag. The microprocessor embedded in the RFID tag interprets the received signal and determines the need for a response signal, i.e., determines whether or not a response signal is warranted. It should be mentioned that data collision is avoided in this system because the RFID tag will respond only when the modulated signal received matches a code stored in the tag and accessed by the microprocessor. If the received signal matches a predetermined code, the tag “wakes-up” and transmits the requested data via a radio circuit. The encoding and the transmission by the tag is handled by either the microprocessor or a state machine that performs the carrier modulation. These circuits are fairly complex and expensive due to the use of microprocessors. These tags generally have some power source in the tag device.
What is needed is a low cost, far field radio frequency identification (RFID) tagging and tracking system. It would be desirable if the RFID tagging and tracking system could utilize lost cost and/or passive RFID tags without encountering data collision. It would also be advantageous if the RFID tagging and tracking system can be utilized in the tracking of inventory and assets for a wide range of applications including, but not limited, to inventory data collection in a warehouse system, tracking of mobile assets, and security monitoring.